Polyethylene terephthalate is widely used upon being molded into a fiber, a film, a sheet, a bottle, a cup or a tray because of its excellent mechanical properties and chemical properties.
Such polyethylene terephthalate can be usually manufactured by using, as starting materials, an aromatic dicarboxylic acid such as terephthalic acid and an aliphatic diol such as ethylene glycol. Concretely, polyethylene terephthalate is manufactured by first subjecting an aromatic dicarboxylic acid and an aliphatic diol to an esterification reaction to form a low-order condensate (ester low polymer) and then subjecting this low-order condensate to a de-glycolation reaction (melt polycondensation) in the presence of a polycondensation catalyst. Also, if desired, after the melt polycondensation, solid phase polycondensation is further carried out, thereby more increasing the molecular weight.
In the manufacturing method of polyethylene terephthalate, an antimony compound or a germanium compound has hitherto been used as a polycondensation catalyst. However, polyethylene terephthalate manufactured by using an antimony compound as a polycondensation catalyst was inferior to polyethylene terephthalate manufactured by using a germanium compound as a polycondensation catalyst with respect to transparency and heat resistance. Also, in particular, for drink bottle applications and packaging materials for foodstuffs, it is also demanded to reduce the acetaldehyde content in the resulting polyethylene terephthalate. Furthermore, there is a thought that the antimony compound has fear in hygiene.
On the other hand, not only the germanium compound is free from fear in hygiene, but also a polyethylene terephthalate molded article manufactured by using a germanium compound as a polycondensation catalyst has good transparency and low acetaldehyde content and oligomer content. But, since the germanium compound is expensive, there was involved a problem that the manufacturing cost of polyethylene terephthalate becomes high.
Furthermore, in recent years, an aluminum compound is proposed as a polycondensation catalyst of polyethylene terephthalate. But, an organoaluminum compound is not comparable to the germanium compound but is still relatively expensive; and it is known that a part of water-soluble aluminum salts are neurotoxic so that fear in hygiene remains, too.
Now, it is also known that a titanium compound has an action to promote a polycondensation reaction of ester; and titanium alkoxides, titanium tetrachloride, titanyl oxalate, orthotitanic acid, and the like are known as a polycondensation catalyst. Moreover, since the titanium compound is not problematic in hygiene and is cheap, a number of investigations for utilizing the titanium compound as a polycondensation catalyst are made.
However, in the case where the titanium compound is used as a polycondensation catalyst, there was involved a problem that the acetaldehyde content or oligomer content in a polyethylene terephthalate chip and its molded article is high as compared with polyethylene terephthalate obtained by using a germanium compound as a polycondensation catalyst. A major component of the subject oligomer is a cyclic trimer of ethylene terephthalate and is hereinafter often referred to as “Cy-3”.
There is a problem that an oligomer present in a polyethylene terephthalate chip or an oligomer formed during polyethylene terephthalate molding adheres as a white powder to rollers of stretching equipment or rollers of heating equipment and stains it; or a problem that an oligomer becomes a foreign matter in a powdered state and stains a dyeing liquid at the time of dyeing working. Also, likewise at the time of fiber manufacture, at the time of film fabrication, there is involved a problem that an oligomer stains rollers of equipment of every kind; or a problem that an oligomer causes a product defect such as so-called dropout in a magnetic tape. Furthermore, at the time of molding of various other molded articles such as a hollow container, an oligomer becomes a white powder to stain a molding die or adhere to a surface of the molded article, whereby a molded article with a normal appearance is not obtainable. In addition, there was a problem that an oligomer generated at the time of stretching processing or heating processing adheres to a die or the like, whereby transparency of a molded article is more remarkably hindered due to transfer.
In order to solve these problems, a method of reducing the oligomer content in polyethylene terephthalate is studied, and a number of proposals have been made. For example, Patent Document 1 and Patent Document 2 propose a method of reducing the oligomer content by a solid phase polycondensation method of heating polyethylene terephthalate in a high vacuum state at not higher than its melting point. Also, Patent Document 3 proposes a method of reducing the oligomer content by a solid phase polycondensation method of heating polyethylene terephthalate in an inert gas atmosphere at a temperature of not higher than the melting point.
In the case of reducing the oligomer content by this method, there is brought an effect for reducing the amount of generation of a white powder with respect to polyethylene terephthalate having a relatively high oligomer content in polyethylene terephthalate. But, with respect to polyethylene terephthalate having a relatively low oligomer content in polyethylene terephthalate, not only the effect for reducing the amount of generation of a white powder cannot be exhibited, but also the amount of generation of a white powder may possibly increase conversely.
Also, in the case where polyethylene terephthalate is used as a packaging resin for foodstuffs, especially drinks, since acetaldehyde contained in polyethylene terephthalate may possibly influence flavor properties of a drink, it is also simultaneously demanded to reduce the acetaldehyde content in polyethylene terephthalate. Patent Document 4 describes that the amount of acetaldehyde and the amount of an oligomer formed at the time of molding by treating polyethylene terephthalate after the solid phase polycondensation with water can be reduced. But, in the case where the polycondensation catalyst is not a germanium compound but an antimony compound, an aluminum compound or a titanium compound, the effect is not exhibited at all.
Incidentally, as a method of reducing the acetaldehyde content or the oligomer content in a polyethylene terephthalate molded article, Patent Document 6 and Patent Document 7 propose a method of adding an alkali metal salt or an alkaline earth metal salt. But, only by this method, the acetaldehyde content in a polyethylene terephthalate molded article is considerably higher than that in polyethylene terephthalate resulting from using a germanium compound as the catalyst.
Also, Patent Document 8 reports a method in which by using a compound obtained by reacting a titanium compound and a monoalkyl phosphate as a polycondensation catalyst, polyethylene terephthalate having a lower acetaldehyde content in a molded article than that at the time of using a conventional titanium catalyst is obtainable. But, even in this method, the acetaldehyde content in a molded article is higher than that in polyethylene terephthalate resulting from using a germanium as the catalyst.    (Patent Document 1) JP-A-48-101462    (Patent Document 2) JP-A-51-048505    (Patent Document 3) JP-A-55-189331    (Patent Document 4) JP-A-3-47830    (Patent Document 5) U.S. Pat. No. 5,017,680    (Patent Document 6) WO 05/023900    (Patent Document 7) JP-A-2004-010657    (Patent Document 8) WO 03/008479